1. Field of the Invention
The present invention relates to an electronic blood pressure measurement device and blood pressure measuring method for measuring blood pressures by wrapping a bladder around a region of a living body. More particularly, the present invention relates to an electronic blood pressure measurement device and blood pressure measuring method which detect the circumferential length of the region for measurement around which an bladder is to be wound.
2. Description of the Background Art
There has existed a conventional electronic blood pressure measurement device as follows. A user winds a cuff enclosing a bladder which is inflated when supplied with fluid such as air around a region for measurement, and then the region for measurement is pressurized or depressurized through the inflation of the bladder. Then, the blood pressure is calculated using a predetermined algorithm based on the pressure within the bladder (cuff pressure) and pulse wave signals acquired at this time. Some blood pressure measurement devices utilize Korotkoff sound instead of pulse wave signals.
However, with such a conventional electronic blood pressure measurement device, it has been difficult to automatically detect the circumferential length of the region for measurement of the user, around which the cuff is to be wound. Therefore, there has been a problem that the accuracy of blood pressure calculation varies depending on the circumferential length of the region for measurement. This is because the accuracy of blood pressure calculation is significantly affected by the relationship between the width of the cuff and the circumferential length of the region for measurement. In order to calculate accurate blood pressure values, it is necessary to press the artery in the region for measurement with an appropriate pressure. In the case where the force for pressing the artery (hereinafter, referred to as a pressing force) is either too large or too small, it is impossible to calculate accurate blood pressure values. In order to provide an appropriate pressing force, it is necessary to employ a cuff with an appropriate width for each circumferential length of the region for measurement.
On the other hand, the cuff is generally configured to have a width applicable to a certain range of circumferential lengths of the region for measurement. Therefore, for some circumferential lengths of the region for measurement, estimated accuracy of blood pressure measurement can not be achieved. Therefore, it is desired to employ a blood pressure measurement process appropriate for the circumferential length of the region for measurement. There have been suggested some conventional blood pressure measurement devices and methods which prepare and selectively employ several types of blood pressure calculation algorisms based on the characteristics of the pulse wave amplitude or the characteristics of Korotkoff sound. However, they have not covered the influences of the circumferential length of the region for measurement.
Therefore, Japanese Laid-Open Patent Publication No. 6-245911 suggested a function of detecting the circumferential length of the region for measurement. This publication suggested a function of inputting the circumferential length from outside, a function of detecting the circumferential length from the value of slide resistance provided in the cuff, and a function of detecting the circumferential length from the time required for raising the pressure within the cuff to a certain level.
The method which inputs the circumferential length from the outside, disclosed in the aforementioned publication, requires the subject to measure and detect the circumferential length in advance and also requires the subject to input the circumferential length every time he performs blood pressure measurement, thus having poor operability. Further, there will be variations in the detected circumferential length depending on the winding condition of the cuff, such as loose winding or tight winding. Therefore, the method using slide resistance or a required time period has not been regarded as a method capable of detecting an accurate circumferential length.